1. Field of the Invention
This invention is related in general to the field of emergency lighting and, in particular, to systems and methods for self testing emergency lighting systems.
2. Description of the Related Art
Emergency lighting is required by most safety codes in the United States. Emergency lights provide temporary lighting in the event of a power failure. During normal operation, power is provided from power mains to operate the lamp and to charge a backup power source (e.g. a battery). When power from the mains is interrupted, the backup power source provides power to the lamp for a limited time (typically 90 minutes).
It is desirable to test emergency lights periodically to ensure proper operation. A typical prior art self test is initiated by a person pressing a button or switch on the lighting unit. A simple voltage and/or current test is performed and a light or buzzer is activated if the test fails.
There are several problems with the prior art. One problem is that safety codes typically require a brief (e.g. 30 second) test every month and a longer (e.g. 90 minute) test each year. The prior art requires a person to initiate, monitor and manually record each of these tests. Consequently, testing is easily neglected, records of the tests are easily lost and costs for personnel to do the testing and track the testing are incurred.
Other problems are associated with self testing of lighting systems which use fluorescent lamps. Fluorescent lamps differ from non-fluorescent lamps in that a high frequency signal is needed to power them. During normal operation AC line power is converted to a higher frequency to efficiently power the fluorescent lamp. When the main power source is interrupted, DC power from the backup power source is converted into a high frequency signal to power the fluorescent lamp.
A problem with the prior art is that only the backup power source and the fluorescent lamp are tested. Other parts and circuits may not be directly tested. This is due to the simple methods used in the prior art to perform the self test. For example, the prior art typically tests the backup power source by simply disconnecting the main power from the system and measuring the voltage across the backup power source. If the voltage drops below a predetermined limit a failure light or LED is activated. This self test method does not detect other failures in the system.
Similarly, the prior art tests fluorescent lamps by simply sensing the current to the lamp. This is done by measuring the voltage across a resistive element and computing current using Ohm s law. There is a correlation between current into a fluorescent lamp and proper lamp operation. If the current is outside of predetermined limits the lamp is determined to be failed and an LED is activated.
Some of the problems with this technique are that a resistive element introduces power loss, interferes with normal operation of the circuit, may require a high tolerance resistive element, may require a high resolution analog to digital (A/D) converter, is unlikely to detect problems in other parts of the system and can fail to detect a failed lamp under some conditions. For example, in the prior art, it is possible that the inverter circuit may consume enough current to "fool" a controller into determining that the lamp and inverter are operating normally when, in fact, either or both are not working properly.
Examples of prior art testing techniques and deficiencies are found in U.S. Pat. No. 5,666,029, issued Sep. 9, 1997, to McDonnell, which is incorporated herein by reference.
Clearly there exists the need for an improved emergency lighting test system and method which automatically perform lighting system tests, automatically keep records of testing, do not require high tolerance components, do not require high resolution A/D converters, are a simple design and provide thorough self testing.